1. Field of the Invention
This invention relates to a method for the production of p-methoxybenzaldehyde. More particularly, it relates to a method for the production of p-methoxybenzaldehyde by the vapor-phase catalytic oxidation of p-methoxytoluene, which method enables p-methoxybenzaldehyde to be produced with high quality.
2. Description of the Prior Art
p-Methoxybenzaldehyde is a useful compound as perfume, brightener for metal plating, and raw material for medicines and has already found quantity consumption in various fields.
For the production of p-methoxybenzaldehyde from p-methoxytoluene, numerous methods have been proposed in the art. Generally, these methods effect the production of p-methoxybenzaldehyde by the liquid-phase reactions: (1) A method which produces p-methoxybenzaldehyde by effecting liquid-phase pressed oxidation of p-methoxytoluene with oxygen in the presence of a lower fatty acid with a soluble salt of manganese used as a catalyst (Japanese Patent Publication SHO 55(1980)-42,974) and (2) a method which produces p-methoxybenzaldehyde by electrolytically oxidizing p-methoxytoluene in the presence of a metal salt (Japanese Patent Laid-Open SHO 55(1980)-85,682) are typical examples.
These methods are invariably unsatisfactory because they have drawbacks of their own from the commercial and economic points of view. To be specific, the method of (1) is unsatisfactory because the yield of p-methoxybenzaldehyde is low and such steps of process as the control of reaction and the recovey of catalyst and solvent impose heavy burdens and the method of (2), though excellent in the yield of the product, is unsatisfactory because the economy of the process has much to be desired in connection with the recovery of the used catalyst, the efficiency of reuse of the recovered catalyst, and the cost of power. Another known method produces p-methoxybenzaldehyde by vapor-phase catalytic oxidation of p-methoxytoluene (Japanese Patent Publication SHO 58(1983)-4,012). This invention only discloses examples of this reaction.
The inventors formerly made a study in search of a method for the production of p-methoxybenzaldehyde by the vapor-phase catalytic oxidation and, consequently, developed a catalyst for producing p-methoxybenzaldehyde commercially advantageously in a high yield without entailing the aforementioned drawbacks (PCT/JP85/00628).
For the production of p-methoxybenzaldehyde by the vapor-phase catalytic oxidation, p-methoxytoluene and a molecular oxygen-containing gas (such as, for example, air) are substantially all the raw materials required. Absolutely no organic solvent is used at any step in the whole production process. The waste liquid occurring in the process at all is practically the whole volume of the water formed by the reaction. Even from the standpoint of the disposal of this waste liquid for the prevention of environmental pollution, this process is highly advantageous both commercially and economically.
When p-methoxytoluene is oxidized in a vapor-phase, however, various by-products occur though in small amounts in addition to the principal product aimed at. It has been demonstrated that these by-products impair the quality of p-methoxybenzaldehyde and that this impaired quality cannot be mended merely by ordinary distillation.
When the aforementioned catalyst is used in the vapor-phase catalytic oxidation of p-methoxytoluene with molecular oxygen or a molecular oxygen-containing mixed gas at elevated temperatures, the conversion of p-methoxytoluene is heightened, the product aimed at, i.e. p-methoxybenzaldehyde, is obtained in high yield and high selectivity, no solvent is required at any step in the whole production process, and the expense for the disposal of the waste liquid from the process is small. Thus, the process proves to be highly satisfactory from the commercial point of view.
Further, when the reaction is carried out in the form of vapor-phase catalysis, the reaction can be effected by a continuous operation, the heat evolved by the oxidation can be recovered by being utilized in heat-exchanging the vapor, and the cost of production can be notably lowered. Thus, the continuous method proves to be highly satisfactory from the economic point of view as well. In this reaction, however, since the product of the oxidation is discharged in the form of a gas of very high temperature from the reaction system, development of a method for efficient collection of the emanating gaseous product poses itself a problem.
Generally, p-methoxybenzaldehyde by nautre is spontaneously oxidized by the oxygen in the air when it is left standing in the air and readily converted into p-methoxybenzoic acid.
When the p-methoxybenzaldehyde produced is in the form of a mixed gas discharged at an elevated temperature from the system of the vapor-phase oxidation, it is gradually oxidized into p-methoxybenzoic acid. The crystals of this acid bring about numerous difficulties such as the clogging of the piping, the hindrance to the operation of the apparatus, and the substantial decrease of the yield of the product during the isolation.
An object of this invention, therefore, is to provide a method for the production of p-methoxybenzaldehyde of high purity.
Another object of this invention is to provide a method for the production of p-methoxybenzaldehyde of high purity by the vapor-phase catalytic oxidation of p-methoxy-toluene.
Yet another object of this invention is to provide a method for the production of p-methoxybenzaldehyde of high purity in a high yield by the vapor-phase catalytic oxidation of p-methoxytoluene.